Table 4. — Major components of the micronekton; actual 

 volumes from table 3, for all areas combined 



phyll a, zooplankton, and micronekton — is posi- 

 tively correlated with the othere in tlie area of 

 cruise TO-58-1 (Blackburn, 1966a). The distribu- 

 tions reflect the fact that physical situations wliich 

 lead to hiofji production of organisms, namely u]i- 

 welling- and vertical mixing by wind over shoal 

 pycnoclines, are better developed along the eastern 

 sides than in other parts of tropical oceans, 

 except along the Equator (Wooster and Reid, 

 1963; Wyrtki, 1966). Crops of chlorophyll a 

 and zooplankton are fairly hig'h in offshore 

 waters along the Equator (Foi-sbergli and 

 Joseph, 1964; Blackburn, 1966b); this paper 

 presents no data on micronekton for those waters, 

 but King and Iversen (1962) found more micro- 

 nekton near the Equator than elsewhere in the 

 central tropical Pacific. In nonequatoria.l olTshore 

 waters the pycnoclines lie deeper and mixed layei-s 

 are thicker (Wyrtki, 1964b); the likelihood of 

 chemical enrichment from Ijelow is diminished, 

 and, if there is such enrichment, part of the result- 

 ing plant crop will be carried by mixing below the 

 compensation depth. 



The standing ci-op of micronekton is not imi- 

 formly high in the coastal region. It tends to be 

 higher in some parts of the region than in others. 

 The sta.ndardized volumes of total micronekton 

 for the 40 onshore stations (table 6) range from 

 113.3 to 2.9 ml. per 1,000 m.% and this group can 

 be divided into an upper two deciles (8 stations) 

 and a lower eight deciles (32 stations). Table 7 

 shows that the eiglit stations with the higlier total 

 volumes also had the higher mean volumes for each 

 of three main components of the micronekton. 



The eight stations with high volumes are num- 

 bers 49, 63, 77, 81, 142, 144B, and 146 of cruise 



TO-58-1, and number 6 of cruise TO-60-2. With 

 the possible exception of station 142, they all occur 

 in localities which are known to be especially pro- 

 ductive of organisms, as a result of physical proc- 

 esses wjiich operate at or a few months prior to 

 the season at which the micronekton was collected. 

 Stations 144B and 146 occur in the coastal upwell- 

 ing region off Baja California (Reid, Roden, and 

 Wyllie, 1958; Blackburn, 1966b) ; stations 77 and 

 81 are in the Gulf of Tehuantepec where vertical 

 mixing occurs over a shoal thermocline (Black- 

 burn, 1962) ; station 49 lies in the "Costa Rica 

 Dome" region where upwelling, from cyclonic 

 flow, takes iplace (Wyrtki, 1964a; Holmes, MS.*) ; 

 station 63 occurs in the coastal upwelling region 

 of the Gulf of Panama (Forsbergh, 1963) ; and 

 station 6 is in the coastal upwelling region off Peru 

 (Wyrtki, 1963; Forsbergh and Joseph, 1964). 



Station 142 is in the mouth of the Gulf of Cali- 

 fornia, which is not known to be as biologically 

 productive as the other areas just mentioned. The 

 mixed layer is <20 m. thick in an average June, 

 however (Wyrtki, 1964b) and was so when the 

 station was occupied in June 1958; the possibility 

 of cliemical and biological enrichment, as a result 

 of vertical mixing, therefore, exists. On the other 

 hand, this station is very close to an island; the 

 higli volume of micronekton (mainly fishes, see 

 table 1), tlierefore, may represent an "island 

 effect." 



The standing crop of cephalopod micronekton 

 appears to diminish polewards from the tropics 

 (fig. 8) even along the coast, but this trend does 

 not hold with the fish and crustaceans. 



Figures 9 to 13 show similar data, from table 5, 

 for some of the major groups of fish and crusta- 

 ceans. Tlie distribution of myctophids (fig. 9), 

 the largest group by volume among the fishes, is 

 broadly similar to that of all fish (fig. 6) . Standing 

 crops decline from onshore to offshore, although 

 not as markedly as crops of some of the other 

 groups (table 6). Myctophids are also well repre- 

 sented in the micronekton off the west coast of the 

 United States (Aron, 1962a; Pearcy, 1964) and in 

 the central Pacific (King and Iversen, 1962) ; it is 

 impossible to make close quantitative comparisons 

 of standing crops from those regions with crops 



' Holmes. Rol>ert W. A contritnition to the plij-sical. cliemicaJ. 

 and biological oceanography of the northeastern tropical Pacitic 

 (Scripps Institution of Oceanography, University of California, 

 357 i)p.). 



86 



U.S. FISH AND WILDLIFE SERVICE 



